In any microwave communication apparatus provided in digital wireless communication system or broadcast system, many micro-strip paths are used in order to reduce the transmission loss of high-frequency power and to provide a circuit pattern well matched in impedance. (See, for example, Jpn. Pat. Appln. KOKAI Publication No. 11-205012 and Jpn. Pat. Appln. KOKAI Publication No. 3-34272. ) The micro-strip paths are signal lines, each comprising a dielectric layer and a copper foil. The dielectric layer is set to the ground potential. The copper foil is laid on the dielectric layer. An air layer is maintained above the substrate.
Examples of microwave communication apparatuses using micro-strip paths are a frequency converter and a power amplifier. Generally, each apparatus has a plurality of modules so that the circuits may be adjusted more easily than otherwise. The modules are first adjusted and then connected together, and therefore constitute an apparatus. The high-frequency line of each module may be changed in propagation mode and be provided as a coaxial connector such as SMA connector, and may therefore have an input/output interface. Alternatively, the high-frequency line of each module may be connected, as micro-strip path, to the high-frequency line of another module.
Of the microwave communication apparatuses, any apparatus in which an intense magnetic field concentrates at the circuit pattern, such as solid-state power amplifier of large power and extremely high frequency, will have a power loss due to transmission loss or reflection loss too large to neglect, if the propagation mode of the micro-strip paths is changed to that of a coaxial connector. Heat is inevitably generated in the apparatus, possibly damaging the circuit or degrading the coaxial connector characteristic. This is because the coaxial cable is insufficiently resistant to power.
This is why the modules are connected, usually by connecting the connection copper foils of the micro-strip paths. A uniform structure should therefore be provided, in which each micro-strip path has a dielectric layer set to the ground potential and a copper foil lies on the dielectric layer. Hitherto, however, that part of the dielectric layer, which lies at the connection part, changes to air, though for a very short distance. The ground distance inevitably changes. This mismatching results in an inductance component, which results in a gap that is too deep to neglect with respect to wavelength, particularly at high frequencies. The gap causes impedance mismatching between the lines. Consequently, trimming must be performed to eliminate the impedance mismatching.